1. Field of the Invention
The invention relates to an electronically controlled device and method for the regulation of pneumatic pressure and to their use in a system for the servocontrol of the speed of the jets coming from a liquid spraying head.
A special application of the invention is that of printing by a continuous jet of ink drops that have to be brought, under constant and precise pressure, in the modulation body of a printer towards a calibrated nozzle.
2. Description of the Prior Art
At present, there are two types of electronically controlled pressure regulators: membrane type regulators and specialized solenoid-valve regulators for which the pressure regulation is obtained by the mechanical equilibrium of a part in motion (membrane-piston)
In an electronically-controlled membrane type regulator, the force that is applied to the membrane is due not to a spring as in a manual control but to an air pressure, obtained by means of a pneumatic potentiometer. The value of the pressure thus regulated ranges from a higher value, called an input pressure, given by a source, and a lower value, called an output or discharge pressure, which is generally the atmospheric pressure. The potentiometer is constituted by an electronic device that incorporates the regulation loop, associated with a nozzle-blade system or with two proportional solenoid valves, or again with a three-way, three-position solenoid valve with high-speed opening and shutting, according to the chosen technology.
In the second type of electronically controlled pressure regulator, the solenoid valve is a three-way solenoid valve receiving an electrical control signal delivered by an electronic regulation device as described in the French patent application FR 2 275 822 by HOERBIGER and the European patent application EP 328 573 by JOUCOMATIC. As a function of this electrical control signal and of the value desired for the regulated pressure, a piston internal to the valve takes different positions providing either for a link between the source pressure and the regulated pressure when the latter is too low with respect to the desired value or for a link between the discharge pressure and the regulated pressure when this regulated pressure is greater than the desired value, or for an imperviousness of the regulated pressure with respect to the source pressure and the discharge pressure.
These electronically controlled pressure regulators firstly are far more costly than manually controlled ones and, secondly, have certain drawbacks such as sensitivity to pollution which dictates intensive filtering in the case of the nozzle-blade system or the oversizing of the elements for systems that require only a small flowrate of air, such as ink-jet printers. Furthermore, the overall efficiency of the pressure regulation system is poor for a requirement of low flowrate since presently used regulators always consume a small amount of air even when the demand is zero. This means that it is necessary to choose the dimensions of the compressor as a function of the consumption of the regulator and not as a function of effective demand. In addition to these drawbacks, there are the problems of hysteresis due to the friction of the moving parts and the problems of instability of the pressure source.
In the field of the invention as used in ink-jet printers, the qualities of printing are closely related to the speed at which the ink is ejected by the nozzles. Now, this speed may be reduced by variations in the pressure of the ink upstream with respect to the these ejection nozzles. Hence, the pressure of the ink should be constantly checked and controlled with high precision. The published French patent application FR 2,652,540 filed on behalf of the applicant, describes a use of compressed air to pressurize the ink circuit of an ink-jet printer, with a manually controlled pressure regulator. The circuit for the supply of ink to the printer, described in this patent application, includes an ink accumulating chamber designed to spray an ink jet to the ejection nozzles. The transfer of ink into this accumulating chamber is done by completely emptying a viscosimeter which is filled with ink from a recovery vessel that is itself connected to an ink container and a solvent container and that checks the viscosity of the ink for the printing. The level of pressure in the accumulating chamber is measured by a needle manometer and this pressure is regulated by a manually controlled regulator that acts on a conduit for the inlet of the pressure given by a compressor. The emptying of the ink from the viscosimeter into the accumulating chamber is done through a calibrated outlet that is dimensioned to limit the ink transfer flowrate in accordance with the response rate of the regulator which should be capable of swiftly dealing with any tendency towards overpressure that would be caused by a sudden arrival of ink in said accumulating chamber.